The present invention relates generally to an electromagnetic pickup which is disposed in the vicinity of a rotating shaft, such as a crankshaft of an internal combustion engine, for sensing the rotation or rotational angle of the rotating shaft, and more particularly, it relates to a filter circuit for such an electromagnetic pickup which is particularly effective to filter or remove low-frequency noise components in an output signal of the electromagnetic pickup which vary in frequency in synchronization with a variation in the frequency of the output signal.
In general, in order to sense the rotational angle or crank angle of the crankshaft of an internal combustion engine, an electromagnetic pickup is disposed in an opposed relation to a ring gear which is mounted on the crankshaft for integral rotation therewith, so that it electromagnetically generates an electrical output signal in the form of a sinusoidal pulse each time it faces any one of teeth formed on the peripheral surface of the ring gear.
The sinusoidal output signal of the electromagnetic pickup thus formed is shaped into a desired rectangular waveform by means of a waveform shaper. In this connection, there is a fear that noise of high and low frequencies can be superposed on the output signal of the electromagnetic pickup due to vibrations of the ring gear and the like. Thus, it is necessary to remove noise components from the output signal of the electromagnetic pickup by means of a low-pass filter and a high-pass filter at the time of waveform shaping.
FIG. 2 is a circuit diagram illustrating a known filter circuit with a waveform shaper for an electromagnetic pickup. In this figure, the illustrated filter circuit includes an input terminal 1 on which an output signal Vi from an unillustrated electromagnetic pickup is imposed, a low-pass filter 2 for filtering or removing high-frequency components in the output signal Vi of the electromagnetic pickup, and a high-pass filter 3 connected in series with the low-pass filter 2 for removing or filtering low-frequency components in the output signal Vi. The low-pass filter 2 includes a resistor 21 connected at one end thereof to the input terminal 1, and a capacitor 22 connected between the other end of the resistor 21 and ground. The high-pass filter circuit 3 includes a capacitor 31 connected at one end thereof to a junction between the resistor 21 and the capacitor 22, and a resistor 32 connected between the other end of the capacitor 31 and ground.
The waveform shaper comprises a comparator 4 having a negative input terminal connected to a junction between the capacitor 31 and the resistor 32 so as to receive a filtered output signal Vf of the high-pass filter 3, a positive input terminal on which a reference voltage Vr is imposed from a reference voltage source 5, and an output terminal 6 for outputting a waveform shaped signal Vo as a final output signal of the electromagnetic pickup.
Now, the operation of the above-mentioned known filter circuit will be described in detail with particular reference to a waveform diagram of FIG. 3. First, as the engine crankshaft rotates together with the ring gear fixedly mounted thereon, the unillustrated electromagnetic pickup generates an output signal Vi having a sinusoidal waveform in synchronization with the crankshaft rotation. The output signal Vi from the electromagnetic pickup generally contains low-frequency and high-frequency noise components superposed thereon. The low-frequency noise components are mainly generated by the crankshaft rotation and have low frequencies corresponding to the rotational speed or number of revolutions per minute of the crankshaft, whereas the high-frequency noise components are generated by other electrical or electronic equipment disposed near the electromagnetic pickup. Thus, in order to remove these noise components, the output signal Vi from the electromagnetic pickup is fed to the input terminal 1 of the filter circuit comprising the low-pass filter 2 and the high-pass filter 3. The low-pass filter 2 serves to attenuate signal components having frequencies higher than a first predetermined higher cut-off frequency f.sub.1, which is set by a circuit constant given by the resistor 21 and the capacitor 22, to thereby remove high-frequency components contained in the output signal Vi. The thus filtered signal Vi is then fed to the high-pass filter 3 where signal components having frequencies lower than a second predetermined lower cut-off frequency fo, which is set by a circuit constant given by the capacitor 31 and the resistor 32, are thereby attenuated and removed. In this respect, the second lower cut-off frequency fo is given by the following equation: EQU fo=1/(2.pi.CR) (1)
where C is the capacitance of the capacitor 31 and R is the resistance of the resistor 32.
In this manner, the output signal Vi of the electromagnetic pickup is subject to filtering so that the high-frequency and low-frequency noise components contained therein are removed to provide a filtered output signal Vf which only contains signal components having desired intermediate frequencies between the upper and lower cut-off frequencies f.sub.1, fo.
The thus filtered signal Vf is input to the negative input terminal of the comparator 4 where it is compared with the reference level Vr at the positive input terminal thereof to generate a waveform-shaped signal Vo in the form of a pulse which is formed by components of the signal Vf lower than the reference level Vr. The waveform-shaped signal Vo is then output from the output terminal 6 of the comparator 4, so that it is finally used for sensing the rotational angle or position of the crankshaft.
With the above-mentioned known filter circuit, however, the frequency and voltage of the output signal Vi generated by the electromagnetic pickup increases in accordance with an increase in the rotational speed or the number of revolutions per minute of the engine, so at the same time, the frequency and voltage of low-frequency noise components superposed on the signal Vi also increase in accordance with the increasing engine rotational speed.
Although the low-frequency noise corresponding to the number of revolutions per minute of the engine is generally filtered and removed by the high-pass filter 3, if the frequency of the low-frequency components increases above the lower cut-off frequency fo, which is set to a constant value irrespective of the engine rotational speed, the low-frequency noise components superposed on the signal Vi are not filtered but are input to the comparator 4.
As a result, the level of the filtered output signal Vf varies in accordance with the varying rotational speed of the engine, as clearly seen from FIG. 3, and in an extreme case, the lowest level of the filtered signal Vf from the high-pass filter 3 can exceed the reference level Vr. In this case, there will be a pulse loss P' in the waveform-shaped output signal Vo from the comparator 4, so no desired or required pulse shape will be obtained, thus resulting in an error in the sensed crankshaft angle or position, for example.
To summarize, in the known filter circuit as described above, the lower cut-off frequency fo is constant irrespective of the engine rotational speed, so if the frequency of the low-frequency noise components due to the engine rotation increases above the lower cut-off frequency fo in accordance with the increasing frequency of the output signal Vi from the electromagnetic pickup, it becomes impossible to filter or remove the low-frequency noise components, making it difficult to obtain a desired or required S/N ratio, particularly in a relatively low part of the intermediate-frequency range of the output signal Vi.